Many continuous mixers of plastic materials depend upon cooling the temperature of the rotors. Normal practice utilizes a rotary joint mechanism with a coolant feed tube extending within a bore in the rotor. This customary practice lacks versatility in that it limits the cooling function to an all or nothing principle along the length of the rotor. In other words, in the prior art, the rotor bore is cooled along its length in an "all" arrangement. Or, the coolant flow is shut off in a "nothing" situation.